In radio signal reception systems, a mixer multiplies an RF (radio frequency) signal amplified using a low noise amplifier by an LO (local oscillator) signal generated by a built-in PLL (phase locked loop) or the like. The mixer thereby frequency-converts the RF signal to an IF (intermediate frequency) signal or baseband signal.
An LO signal is given as a rectangular wave. However, the rectangular wave includes not only a fundamental wave component f but also odd-number-order harmonic components 3f, 5f, 7f, and so forth. For that reason, an RF signal which is not a desired signal is also mixed in the output of the mixer in the same frequency band as that of the desired signal. For example, in a direct conversion system, not only a desired wave fd but also components of 3fd, 5fd and 7fd are mixed in as the same baseband signal as the desired wave fd.
A harmonic rejection mixer (hereinafter, referred to as “HRM”) is used to suppress harmonic components. An HRM includes a plurality of switching devices that mix an RF signal and an LO signal, a gain device that determines a gain of the output of each switching device and an adder that adds up outputs of the gain devices.
The phase of the LO signal given to the switching device corresponding to the gain of the gain device is set to a special value that can suppress harmonic components. For example, a 3-phase HRM is provided with 3 switching devices. The gains of the gain devices connected to the respective switching devices are set to a ratio of 1:√2:1. At this time, the phases of the LO signal are given while being shifted by 45° each such as 0°, 45° and 90°. The outputs of the switching devices are added up by an adder. By so doing, the third-order and fifth-order harmonics are suppressed.
In general, when N=1, 2, 3, 4, . . . , odd-number-order harmonics such as third-order to (2N+1)th-order harmonics are suppressed as follows. That is, in a (N+1)-phase HRM, the gain ratio is assumed to be sin(1×180°/(N+2)):sin(2×180°/(N+2)): . . . :sin((N+1)×180°/(N+2)). The phase difference between given LO signals is assumed to be 180°/(N+2).
Wireless sensor systems expected to be in widespread use in the M2M (Machine to Machine) field in the future are estimated to use a plurality of frequency bands of 400 MHz to 5.2 GHz. Therefore, the HRM is required to support wideband reception. On the other hand, as the number of phases increases and as the receiving frequency increases, the HRM needs to generate and drive multi-phase LO signals at a high frequency, which results in an increase in current consumption.
Under such circumferences, when attempting to support wideband reception, the HRM encounters a problem in that there may be waste in current consumption depending on the number of phases. This problem will be described with a case corresponding to reception of 400 MHz to 5.2 GHz as an example. That is, in order to support reception of 400 MHz, it is necessary to suppress harmonics of up to the 13-th order. Thus, the HRM operates as a 7-phase HRM. On the other hand, in order to support reception of 1 GHz, harmonics of up to the fifth order may be suppressed. Thus, it is sufficient that the HRM operates as a 3-phase HRM. That is, upon receiving 1 GHz, if the HRM operates as a 7-phase HRM, there is a problem in that there may be waste in current consumption.
NPL 1 discloses a technique for solving such a problem, for example. According to the technique of NPL 1, current consumption is suppressed by reducing the number of phases of an HRM when a receiving frequency is high.